A nebulizer is a device that can be used for converting a liquid into droplets. For some applications, it may be desirable to nebulize a relatively high-temperature liquid (i.e., above 100° C.) into small-diameter droplets (i.e., less than 10 μm). For example, one such application exists in the field of plasma processing. Specifically, in plasma processing, it may be desirable to nebulize a material with a high melting temperature into small-diameter droplets that can then be further heated to create a plasma of the material. Indeed, there are numerous other applications wherein the nebulizing of high-temperature liquids may be required. For example, in powder metallurgy it may be desirable to nebulize a molten solder or a dry molten sodium hydroxide (NaOH), which has a melting temperature of 320 degrees Centigrade (320° C.), into droplets that have diameters in the range of one to three microns (1-3 μm).
One type of well known nebulizer is a so-called ultrasonic nebulizer. In the operation of an ultrasonic nebulizer, acoustic waves having an ultrasonic frequency are directed to a point on the surface of the liquid that is to be atomized. At the point on the surface of the liquid where these ultrasonic waves converge, they will produce capillary waves that oscillate at the frequency of the ultrasonic waves and have amplitudes that correspond to the energy that is in the ultrasonic waves. It then happens, at sufficiently large amplitudes (i.e., high energy ultrasonic waves), that the peaks of the capillary waves tend to break away from the liquid and be ejected from the surface of the liquid in the form of droplets. In this process, the diameter of the droplets that are formed will generally be inversely proportional to the frequency of the capillary waves.
A device that is often used for generating ultrasonic waves in an ultrasonic nebulizer is a piezoelectric transducer. As is well known, a piezoelectric transducer will vibrate and generate ultrasonic waves in response to an applied electric field. Of particular importance, insofar as nebulizers are concerned, is the fact that piezoelectric transducers can operate at relatively high frequencies and, thus, can be used to nebulize a liquid into droplets that have relatively small diameters. Piezoelectric transducers, however, have limited operational temperature ranges. More specifically, piezoelectric transducers are typically made of piezoelectric ceramic materials that lose their piezoelectric properties above the Curie temperature of the material. Consequently, at high operational temperatures, most piezoelectric materials will no longer vibrate in response to an applied electric field. It happens that for most piezoelectric ceramic materials, the Curie temperature is less than three hundred degrees Centigrade (300° C.). In general, most piezoelectric transducers will not effectively operate above about one hundred degrees Centigrade (100° C.).
For the effective operation of an ultrasonic nebulizer that incorporates a piezoelectric transducer, it is obviously desirable to transfer as much energy as possible from the piezoelectric material to the point where the liquid is being nebulized. An effective way to do this is for the transducer to be in contact with the liquid. However, as discussed above, when high-temperature liquids are to be nebulized, the conductive transfer of heat from the liquid to the transducer can adversely affect the operation of the transducer. This fact has required that the liquid be at a relatively low temperature in order for the transducer to function properly. Accordingly, the adverse effect that high temperatures have on piezoelectric materials has effectively limited their use in nebulizers.
In attempts to overcome the high-temperature issue noted above, one type of ultrasonic nebulizer that has been employed to nebulize high-temperature liquids is a rod nebulizer. In a rod nebulizer, the piezoelectric transducer is attached to one end of the rod, and the free end of the rod is placed in contact with the high-temperature liquid that is to be nebulized. When activated, the piezoelectric transducer causes the free end of the rod to vibrate at its resonant frequency. The resultant vibrating action nebulizes the high-temperature liquid into droplets. A rod nebulizer, however, has a limited operational frequency range that is dependent on the length of the rod. Furthermore, the higher frequencies that are needed for most applications require shorter rods. Thus, heat transfer through the rod to the transducer, again, becomes a problem.
In light of the above, it is an object of the present invention to provide a device and method for nebulizing high-temperature liquids (e.g. liquids with temperatures above three hundred degrees Centigrade) into small-diameter droplets. Another object of the present invention is to provide a device and method for distancing a piezoelectric transducer from a high-temperature liquid in a nebulizer to maintain the temperature of the transducer at an operational temperature. Yet another object of the present invention is to provide a device and method for nebulizing a liquid that is relatively easy to manufacture, is simple to use, and is comparatively cost effective.